RISC-V: add support for restartable sequences

Add support for RSEQ, restartable sequences, for RISC-V.  This also adds
support for the related selftests.

Note: the selftests require a linker with 3e7bd7f2414 ("RISC-V: Fix
linker problems with tls copy relocs."), which was first released in
2.33 (from 2019).

* palmer/riscv-rseq:
  rseq/selftests: Add support for RISC-V
  RISC-V: Add support for restartable sequence